As a background for the recent increase in communication traffic, the demand for optical communication/transmission apparatuses is increasing. Not only for optical repeating nodes introduced with backbone networks, but also recently, the introduction of optical transmission apparatuses for local networks is being actively performed. Furthermore, optical networks are also being formed for subscriber loops. In this manner, optical communication systems bear an important role with respect to world information networks.
As a typical optical communication system, an optical amplification repeating transmission system which has high reliability at a low cost and realizes large-capacity and long-distance transmission by arranging on a transmission line optical repeating nodes provided with wavelength division multiplexing (WDM) optical amplifiers such as for example erbium-doped fiber amplifiers (EDFA), becomes mainstream.
In such an optical amplification repeating transmission system, if the repeating distance between nodes becomes long, the losses of the transmission line increase. More specifically, the loss per unit length of the transmission line is generally around 0.2 dB/km, and the loss of transmission line of one repeating section increases corresponding to the repeating distance. Furthermore, in the case where various functional optical components are arranged on the transmission path, the transmission losses of these functional optical components add up so that the span losses become even greater. Therefore, the input level of the signal light to the WDM optical amplifier of each optical repeating node becomes smaller, and the OSNR (Signal-to-Noise Ratio) that expresses the intensity ratio between the signal light and the noise light is reduced, so that there is the possibility of deterioration in transmission characteristics. As one means for avoiding this deterioration in the transmission characteristics due to the increase in the span losses, there is known for example a distributed Raman amplifier (DRA) that supplies pump light to a transmission line arranged on an input side of a WDM optical amplifier, and Raman amplifies the signal light propagating through the transmission line using the amplification affect due the induced Raman scattering affect (refer for example to M. Takeda et al., “Active Gain-Tilt Equalization by Preferentially 1.43 μm- or 1.48 μm-Pumped Raman Amplification”, OAA '99, ThA 3-1, 1999) and this is realized.
In a system which uses this Raman amplification in combination, under system requirements which generate a large span loss, a high gain is necessary for the signal light amplification device (EDFA, DRA or the like). Therefore, the power of the noise light generated from the amplification device also increases, so that the proportion of the signal light power with respect to the noise light power (OSNR) is reduced. Furthermore, for example as illustrated in FIG. 10, as the signal wavelength number included in the WDM light becomes less (the signal light is for one wave in the example of FIG. 10), the total power of the signal light is reduced, and hence the above mentioned proportion becomes smaller. Therefore, as one incident which becomes a problem, there is the reduction in the control accuracy at the control device involved with level adjustment of the signal light (for example the various control units equipped with optical amplifiers or variable optical attenuators), arranged downstream of the amplifying device that produces the noise light.
More specifically, in a common configuration of the above control device, a part of the light that includes the signal light and the noise light sent from the upstream side is branched in a branching coupler, and the total power of the branched light is monitored using a photodetector. Then, based on signal wavelength number information for which notification is received from an optical supervisory channel (OSC) or the like of the optical transmission system, the signal light power per one channel is obtained by dividing the monitored total power by the signal wavelength number, and the gain of the optical amplifier or the attenuation amount of the variable optical attenuator is controlled so that this becomes a desired level. As a matter of course, since control of the signal light is the object, if the noise light power with respect to the signal light power becomes large, the control accuracy deteriorates. In the case where the signal light power is not controlled to a normal level, there is the possibility that deterioration of transmission characteristics occurs. That is to say, if the signal light level becomes large, then waveform deterioration of the signal light due to the non linear affect on the transmission line occurs. Furthermore, if the signal light level is reduced, waveform deterioration of the signal light due to the influence of the OSNR drop occurs, so that there is a high possibility of reception error.
As a conventional technique for suppressing the above reduction in control accuracy due to noise light, for example it has been proposed to provide a cutoff section for transmitting/cutting off signal light to an input side of a transmission line, and vary the power of the Raman amplification pump light supplied to the transmission line, and change the status of the cutoff section depending on the variation of the pump light power, and then separately detect the signal light power and the noise light power based on a monitor value of the pump light power and a monitor value of the input light power to the downstream device, and correct the noise light (for example refer to International Publication Pamphlet No. WO 2004/105275).
Furthermore, for example a technique has also been proposed to monitor the power of the pump light supplied to the transmission line, compute the Raman gain from the monitor result, and calculate the generation amount of noise light based on this Raman gain and reflect this in the control (refer for example to Japanese Laid-open Patent Publication No. 2006-189465).
However, in the above conventional technique, in relation to the technique where a signal light cutoff section is provided and the noise light power is detected, since this is a method of cutting off the signal light on the input side of the transmission line, it is difficult to monitor the noise light power while in service. That is to say, there is a problem in that the influence of the noise light generated due to Raman amplification is monitored in real time, and this monitor result cannot reflect on control in the downstream control unit.
Furthermore, in relation to the technique for computing the generation amount of Raman gain and noise light power from the monitor results of the pump light power, this is effective in the case where there is a one-to-one relationship between the pump light power and the Raman gain. However, in general, the above relationship changes corresponding to the complex requirements of the transmission line (for example the type of transmission line, the coefficient of loss, the effective cross-sectional area, the length, and so forth). Moreover with regards to the relationship between the monitor value of the pump light power, and the pump light power actually supplied to the transmission line, it is difficult to consider that this continues as a one-to-one relationship during operation. Consequently, there is a problem in that it is difficult to monitor with good accuracy in real time the power of the noise light generated due to the Raman amplification. In contrast to this, for example it is considered to store the above relationship corresponding to the requirements of the transmission line in a database, and extract a relationship corresponding to the system requirements that is notified by a supervisory signal or the like, from the database, and compute the noise light power. However, in order to realize highly accurate computation, it is necessary to store an enormous amount of data, and it is not easy to realize a database of such an enormous size. Even if a database can be prepared temporarily, processing that takes time to select data suitable for the system requirements from amongst the data is necessary. Therefore there is a problem in that it is difficult to correspond to high speed control in the downstream control unit.